1. Field of the Invention
This invention relates to a laser system capable of high precision cutting, suitable for silicon wafer dicing. The laser system uses a diode pumped laser with a short pulsewidth and high pulse repetition frequency. The laser system is also suitable for other purposes, such as surface processing and milling.
2. Description of Related Art
Dicing is a cutting operation that is used to separate integrated circuits (ICs) from their parent wafers. The dicing of IC patterned silicon wafers currently relies on a technology initially developed in the early 1970's. The dicing step is performed by a two-and-one-half inch diameter diamond-impregnated circular saw blade running at extremely high rpms, lubricated by large amounts of water. Material removal occurs through the creation of micro-cracks at the point of contact between the diamond particles and the substrate. As micro-cracks intersect, debris is generated and cutting takes place.
This method of dicing introduces variables that are disadvantageous to the cutting process. The use of diamond embedded blades requires the use of water for lubrication, cooling, and washing away silicon particles. Water in contact with the processed wafer surface creates substantial contamination problems for the ICs and waste water disposal issues, as well as reduces throughput due to the time necessary to dry the wafers. The problem of blade wear requires frequent dressing and changing of the blades, resulting in down time of equipment, and the increased possibility for reproducibility problems.
Current dicing equipment is restricted to uni-directional cutting because the cutting head must be fixed, with movement provided by the substrate positioning stage. The circular blade also restricts the flexibility of cutting geometry to straight lines and 90 degree corners. In addition, the width of the blade restricts the kerf, or cut width, to approximately 25-37.5 microns. The ability of future dicing blade technology to reduce this kerf, and thus increase IC population per wafer, is limited.
The depth of damage and the size of the kerf depend on many different variables, some relating to the properties of the silicon and others to the capabilities of the circular cutting device. A short list of the variables includes the chemical properties of the silicon, blade parameters such as vibration, relative motion of the blade and workpiece, up or down cutting, and the chemistry and delivery of lubricants. The level of damage to the silicon wafer is characterized by the length and distribution of micro-cracks, and the penetration of dislocations. Most importantly, the length and distribution of micro-cracks can influence the future reliability of the IC.
Currently, dicing saw technology allows for a maximum speed of travel across the silicon wafer of approximately 50-75 mm/s. The processing speed of silicon wafers is of extreme importance because it directly translates to higher throughput of the processed substrate. Unfortunately, cut quality is difficult to maintain at high processing speeds due to increased chipping of the substrate and reproducibility problems relating to increased blade wear. In general, attempts to increase the throughput of the process by increasing the cutting speed have resulted in either inferior cut quality, unacceptable blade wear, or both.
The requirement for high cut quality and speed demands the inclusion of many expensive internal components in the current generation of dicing equipment. A large expense relates to the need for very low vibration of the 60,000 rpm blade. Due to the brittle nature of the silicon substrate, the slightest vibration can induce fractures. To minimize vibrational perturbations, the blades are mounted on ultra-expensive air-bearing spindles. Also, the electric inverter needed to operate a motor at high and precise rpms is expensive.
Since the infancy of dicing technology, various academic and industrial labs have attempted to use laser devices to dice silicon wafers. Unfortunately, the results have been less than favorable. In the 1970's, laser dicing of silicon wafers was tried and repudiated. The primary shortcomings were cut quality (surface smoothness) and re-solidified material on the wafer surface. The primary culprit was the pulsewidth of the irradiated energy: a microsecond was far too long a period to prevent destructive heat dissipation into a significant portion of the area immediately surrounding the cut. The difficulty with these longer pulsewidths (actually 1 nanosecond and longer) is that the pulse interacts with the material for too long a time, and the heat affected zone penetrates deep into the substrate.
Recently, one of the largest U.S. manufacturers of dicing saws analyzed the comparative results of conventional blade dicing and laser dicing. The study found that the depth of damage was greater for the laser diced wafers resulting in a lower fracture strength of the silicon substrate. The report stated that the increased damage created by the laser was a result of thermally induced stresses.
These stresses result from excessive heat deposition into the wafer causing collateral damage to the areas of the wafer proximal to the cut. This collateral damage is characterized by micro-cracking and spatter, both entirely unacceptable attributes. The individual silicon chips, or die, that are produced by the dicing of the wafer cannot have micro-cracks exceeding 1 micron in length. Longer cracks are potential future failure sites, as these cracks might eventually propagate due to thermal cycling or vibrations encountered in the field.
Near infra-red (IR) lasers are used for high speed applications due to their ability to generate greater power than their ultra-violet (UV) counterparts. Unfortunately, near IR wavelengths propagate deep into silicon substrates before being absorbed because silicon is semi-transparent at these wavelengths. This results in excess energy deposition deep into the substrate. UV lasers have been used in the past to alleviate this problem, because silicon is highly absorptive to UV wavelengths. However, UV lasers can not develop sufficient power to process silicon at high speeds.
Recently, researchers have found that the deep absorption depth problem associated with near-IR lasers in silicon can be solved using ultra-short pulse widths. Ultra-short pulse widths produce the formation of a near solid density plasma, which acts as a highly energy absorptive layer. The absorbing properties of the plasma prevent the transmission of excess energy deep into the substrate.
Ultra-short pulses alone will not make a feasible laser silicon dicing system. Pulse repetition frequency (PRF), pulse energy and beam quality are very important parameters. Advantages of high pulse repetition frequency include smoother and higher quality cuts, as well as the linear relationship between PRF and processing speed.
It is highly desirable in the semiconductor manufacturing industry to increase the speed of the wafer dicing process, as this results in a significant cost benefit due to higher throughput. However, it is imperative that the quality of the cuts not be compromised by an increase in speed. The current dicing method has not allowed an increase in cutting speed without an unacceptable loss in cut quality.
There is a need for a laser with high average power, short pulsewidth, and high pulse repetition frequency suitable for silicon wafer dicing. There is a need for a laser suitable for silicon wafer dicing with high cutting speeds.